Defibrillation/cardioversion is a technique employed to counter arrhythmic heart conditions including some tachycardias in the atria and/or ventricles. Typically, electrodes are employed to stimulate the heart with electrical impulses or shocks, of a magnitude substantially greater than pulses used in cardiac pacing.
Defibrillation/cardioversion systems include body implantable electrodes and are referred to as implantable cardioverter/defibrillators (ICDs). Such electrodes can be in the form of patches applied directly to epicardial tissue, or at the distal end regions of intravascular catheters, inserted into a selected cardiac chamber. U.S. Pat. Nos. 4,603,705; 4,693,253; 4,944,300; and 5,105,810, the disclosures of which are all incorporated herein by reference, disclose intravascular or transvenous electrodes, employed either alone or in combination with an epicardial patch electrode. Compliant epicardial defibrillator electrodes are disclosed in U.S. Pat. Nos. 4,567,900 and 5,618,287, the disclosures of which are incorporated herein by reference. A sensing epicardial electrode configuration is disclosed in U.S. Pat. No. 5,476,503, the disclosure of which is incorporated herein by reference.
In addition to epicardial and transvenous electrodes, subcutaneous electrode systems have also been developed. For example, U.S. Pat. Nos. 5,342,407 and 5,603,732, the disclosures of which are incorporated herein by reference, teach the use of a pulse monitor/generator surgically implanted into the abdomen and subcutaneous electrodes implanted in the thorax. This system is far more complicated to use than current ICD systems using transvenous lead systems together with an active can electrode and therefore it has o practical use. It has in fact never been used because of the surgical difficulty of applying such a device (3 incisions), the impractical abdominal location of the generator and the electrically poor sensing and defibrillation aspects of such a system.
Recent efforts to improve the efficiency of ICDs have led manufacturers to produce ICDs which are small enough to be implanted in the pectoral region. In addition, advances in circuit design have enabled the housing of the ICD to form a subcutaneous electrode. Some examples of ICDs in which the housing of the ICD serves as an optional additional electrode are described in U.S. Pat. Nos. 5,133,353; 5,261,400; 5,620,477; and 5,658,321 the disclosures of which are incorporated herein by reference.
ICDs are now an established therapy for the management of life threatening cardiac rhythm disorders, primarily ventricular fibrillation (V-Fib). ICDs are very effective at treating V-Fib, but are therapies that still require significant surgery.
As ICD therapy becomes more prophylactic in nature and used in progressively less ill individuals, especially children at risk of cardiac arrest, the requirement of ICD therapy to use intravenous catheters and transvenous leads is an impediment to very long term management as most individuals will begin to develop complications related to lead system malfunction sometime in the 5-10 year time frame, often earlier. In addition, chronic transvenous lead systems, their reimplantation and removals, can damage major cardiovascular venous systems and the tricuspid valve, as well as result in life threatening perforations of the great vessels and heart. Consequently, use of transvenous lead systems, despite their many advantages, are not without their chronic patient management limitations in those with life expectancies of >5 years. The problem of lead complications is even greater in children where body growth can substantially alter transvenous lead function and lead to additional cardiovascular problems and revisions. Moreover, transvenous ICD systems also increase cost and require specialized interventional rooms and equipment as well as special skill for insertion. These systems are typically implanted by cardiac electrophysiologists who have had a great deal of extra training.
In addition to the background related to ICD therapy, the present invention requires a brief understanding of automatic external defibrillator (AED) therapy. AEDs employ the use of cutaneous patch electrodes to effect defibrillation under the direction of a bystander user who treats the patient suffering from V-Fib. AEDs can be as effective as an ICD if applied to the victim promptly within 2 to 3 minutes.
AED therapy has great appeal as a tool for diminishing the risk of death in public venues such as in air flight. However, an AED must be used by another individual, not the person suffering from the potential fatal rhythm. It is more of a public health tool than a patient-specific tool like an ICD. Because >75% of cardiac arrests occur in the home, and over half occur in the bedroom, patients at risk of cardiac arrest are often alone or asleep and can not be helped in time with an AED. Moreover, its success depends to a reasonable degree on an acceptable level of skill and calm by the bystander user.
What is needed therefore, especially for children and for prophylactic long term use, is a combination of the two forms of therapy which would provide prompt and near-certain defibrillation, like an ICD, but without the long-term adverse sequelae of a transvenous lead system while simultaneously using most of the simpler and lower cost technology of an AED. What is also needed is a cardioverter/defibrillator that is of simple design and can be comfortably implanted in a patient for many years.
Ventricular tachycardia (“VT”) is a relatively common and serious cardiac rhythm disorder in patients with serious heart disease. VT may lead to symptoms of shortness of breath, chest pain, loss of consciousness and even death. VT, like ventricular fibrillation, can usually be terminated with a high-energy electrical shock. Unlike ventricular fibrillation, however, VT, especially VT with a regular rate and ECG pattern, can, on some occasions, be terminated with a type of pacing called anti-tachycardia pacing (“ATP”). Cardioversion/defibrillation of VT is done with high-energy shocks, which usually is very uncomfortable, albeit potentially life-saving. ATP, on the other hand can stop VT painlessly with relatively low energy electrical stimuli, similar to standard pacing pulses as used in standard pacemakers. Conventional ICDs often use ATP to avoid the need to shock-terminate VT, in some cases of VT. ATP can be of several types, all of which use electrical stimuli delivered at rates faster than the intrinsic or native VT rate in order to “over-drive” the VT and take control of the electrical mechanism of the heart causing the VT. ATP is possible because VT is similar in nature to ATP, except one is intrinsic to the heart and one is artificially generated with a pacemaker. ATP merely over-drives the patient's rapid pacemaker (i.e., VT), usurps control of the heart rhythm, and then abruptly ceases to control the heart rhythm. This process can all be directed by the physician who programs the ATP modes in the implantable device. Once ATP usurps control of the heart rhythm from the patient's VT mechanism, and once ATP stops, the patient's rhythm can then return to normal.
ATP will work in about 75% of the cases with monomorphic VT (i.e., VT having a regular rate and ECG pattern) and being at rates typically under 250 beats per minute. There are several types of methods for over-driving the patient's VT. Most derive from two general forms. One is called “burst” ATP. The other is termed “ramp” ATP.
In burst ATP, the device is asked to deliver pacing stimuli of a certain number, typically around 5-15, at a rate that is modestly to moderately faster than the rate of the patient's VT. This rate is fixed for any attempt and the interval between pacing stimuli is constant. In some forms of burst pacing, if the first attempt fails to terminate VT, the next attempt may be slightly faster, according to pre-selected parameters as dictated by the software of the device or by the physician. The physician may program a maximum rate that could be allowed with ATP.
In ramp ATP, the device is asked to deliver pacing stimuli of a certain number, again typically around 5-15, also starting at a rate that is modestly to moderately faster than the rate of the patient's VT. Unlike burst ATP, however, ramp ATP does not use a fixed rate for each attempt. In ramp ATP, for any given attempt, the interval between pacing stimuli gets progressively shorter, i.e., the pacing rate gets progressively faster. As in some forms of burst ATP, if the first attempt of ramp ATP fails to terminate VT, the next attempt may be slightly faster, add more pacing stimuli, change the coupling intervals between pacing stimuli and/or add more attempts as guided by pre-selected parameters as dictated by the software of the device or by the physician.
Conventional ICDs can also use a variety of combinations of these two basic forms of ATP, but the principle remains the same. Stimulate the heart with a low-energy pacing pulse that can over-drive the patient's VT which can, thereafter stop according to device and physician directions to leave the heart with its normal rhythm that existed before the onset of the VT episode.